1. Field of the Invention
The present invention relates to a read operation of a multi-level nonvolatile semiconductor memory.
2. Description of the Related Art
In a nonvolatile semiconductor memory in which one cell unit is configured by memory cells, e.g., a NAND flash memory (e.g., see the specification of U.S. Patent Application Publication No. 2004/0109357), with an increase in the packing density of a memory cell array, several problems arise, caused by the wiring resistance of word lines in one cell unit and parasitic capacitance between the word lines.
One of the problems is a prolonged set up term of word lines upon reading.
The term “set up term of word lines” as used herein refers to a term from when a selected read potential begins to be supplied to a selected word line until sensing begins.
The term “selected read potential” refers to a potential to which on/off of a selected memory cell serving as a target of a reading is determined based on data in the memory cell. The expression “sensing begins” refers to that the potentials of bit lines begin to be brought to values based on data in selected memory cells or data in memory cells begin to be sensed by a sense amplifier.
Furthermore, when it is simply said “reading”, it includes both normal reading and verify reading. The term “normal reading” refers to that data in memory cells are output external to the chip. The term “verify reading” refers to that reading is performed to verify whether data is written properly upon writing.
When the packing density of a memory cell array increases, word lines in one cell unit become thinner, narrower, and longer, increasing wiring resistance. In addition, the distances between the word lines become narrower, increasing parasitic capacitance.
Under such circumstances, it takes a certain amount of time for a non-selected read potential to be thoroughly transmitted from one end (the nearest end to a word line driver) of a non-selected word line to the other end (the farthest end from the word line driver), that is, a waveform of the rise of the potential of the other end of the non-selected word line is gradual.
The term “non-selected read potential” refers to a potential which turns on a non-selected memory cell which does not serve as a target of reading, regardless of the threshold voltage of the memory cell and is larger than a selected read potential to be supplied to a selected word line.
Hence, due to the rise of the potential of the other end of the non-selected word line, the potential of the other end of the selected word line rises to near the non-selected read potential by capacitive coupling between the selected word line and the non-selected word line adjacent thereto.
In addition, as described above, since the waveform of the rise of the potential of the other end of the non-selected word line is gradual, even after a selected read potential is supplied to one end of the selected word line, the potential of the other end of the selected word line is influenced by the potential of the other end of the non-selected word line and thus maintains a potential near the non-selected read potential.
Therefore, it takes a very long time for the potential of the selected word line to drop from the potential near the non-selected read potential and finally settle to the selected read potential.
Meanwhile, to prevent erroneous reading, sensing cannot begin until the potential of the other end of the selected word line reaches the selected read potential.
Due to such reasons, the set up term of word lines is prolonged.
Particularly, in a multi-level nonvolatile semiconductor memory having memory cells, each storing three or more level data, a selected read potential to be supplied to a selected word line has two or more values. That is, as the number of data to be stored in one memory cell increases, the difference between the value of a non-selected read potential and the lowest value of the selected read potential becomes larger.
Therefore, when the selected read potential has the lowest value, a term during which the potential of the other end of the selected word line changes from near the non-selected read potential to the selected read potential is longest.
The set up term of word lines is fixed to the above-described longest term, regardless of the value of the selected read potential.
Accordingly, in a multi-level nonvolatile semiconductor memory, a prolonged set up term of word lines due to an increase in the packing density of a memory cell array is particularly noticeable.
Such a prolonged set up term of word lines becomes an inhibiting factor to a high speed of a read operation.